The present invention relates to seismic transducers. More particularly, the invention relates to seismic transducers for generating horizontally oriented shear forces and motions.
Shear waves propagate in the rigid matrix component of a granular medium and, therefore, are not influenced in a first-order way by the fluid content in the pores of the medium. For example, shear waves will propagate in essentially the same manner and at the same velocity in a granular material such as soil regardless of whether the medium is saturated with water or not. Compressional waves are directly dependent on the bulk modulus of the fluid in the pores of a granular medium such as soil and, therefore, are strongly influenced in amplitude and propagation velocity by ground water saturation conditions.
In many geological field settings of interest, the ratio of shear wave velocity to compressional wave velocity is in the range of 0.25 to 0.35, indicating that the wavelength of shear waves is less than the wavelength of compressional waves by this same velocity ratio factor. Correspondingly, the source frequency required to produce a shear wave having a given wavelength is reduced by the same velocity ratio factor. For example, in an earth medium having a shear wave velocity of 150 m/s and a compressional wave velocity of 500 m/s, the wavelength associated with a 500 Hz shear wave is .lambda..sub.s =150/500=0.30 m (approx. 12 in.) and the wavelength associated with a compressional wave at the same frequency is .lambda..sub.p =500/500=1.0 m (approx. 40 in.). Thus, shear waves at 500 Hz have the potential to detect localized cavity targets as small as 15-30 cm (6-12 in.) in diameter whereas compressional waves at 500 Hz have a potential cavity detection threshold size limit of 50-100 cm (20-40 in.). Higher operating frequencies can yield improved resolution and stronger reflected signals from these size targets. However, absorptive attenuation in granular media such as soil increases exponentially with frequency and, hence, attenuation effects and associated propagation distance limitations will be greater for compressional waves than shear waves when the same threshold target size detection and resolution is required.
Seismic shear waves offer potential advantages over compressional waves in shallow geophysical and geotechnical applications because of their lower wave propagation velocity and their transverse particle motion polarization. The lower propagation velocity of shear waves provides an improved ability to detect and resolve reflections from small localized anomalies in the medium. The transverse particle motion of shear waves provides selective reflectivity from planar surfaces either to minimize shear-to-compressional wave conversion at reflecting interfaces as occurs when the particle motion is parallel to the interface or to minimize coherent interference such as associated with surface waves (commonly referred to as `ground roll`) by using horizontally polarized shear waves radiated in the azimuthal direction from the source to the detectors. Another important advantage of shear waves is the fact that shear waves do not propagate efficiently in media that have low or negligible shear modulus such as non-rigid materials (i.e., mud or water). For this reason, shear waves provide strong reflection responses from air-, water-, or mud-filled cavities such as washout cavities or open fractures in earth materials or in structures made of concrete or other engineering materials.
Shear wave vibrators have been developed for relatively deep seismic geophysical surveys where the reflection targets are large-area layer interfaces and geologic faults at typical depths of a few hundred meters to several thousand meters. The shear wave vibrator sources used for these applications are generally large in size and are operated at substantial power levels by hydraulic-pressure-driven force coupling mechanisms to produce shear waves which are generally limited to the frequency range of about 5-200 Hz.
Although the existing shear wave sources are capable of generating predominantly shear stresses and radiated shear waves at the ground coupling point, these sources also generate a noticeable amount of compressional radiation which, in many cases, causes unwanted interference. Further, deep shear wave seismic exploration applications require high-power source signals (shear forces in the range of about 2,000-20,000 lbf) and, therefore, the presently available shear wave vibrator source systems are generally very large and expensive. Thus, the existing technology in seismic shear wave seismic vibrator sources is not suitable for shallow high-resolution applications directed primarily toward shallow resource exploration, subsurface environmental surveys, and indirect sensing and detection of ground geotechnical conditions and anomalies. In comparison with this existing technology, a shear wave seismic source system appropriate for these shallow applications would preferably operate at relatively high frequencies (typically 200-1,600 Hz) in order to resolve relatively small target details and need only be capable of operating at relatively low shear forces (typically 100-500 lbf) to provide useful results at shallow depths and short propagation path lengths.
Therefore, the object of this invention is to create a new seismic shear wave source capable of generating shear waves uncontaminated by residual compressional waves at frequencies typically up to 1,600 Hz and operating at only moderate shear driving forces typically up to 500 lbf A shear wave vibrator source of this type will be appropriate for shallow geophysical and geotechnical applications and will be matched in size, cost, and mobility appropriate for such shallow field surveys.